- RAW FILE FORMAT -

The RAW file format is digital photography's equivalent of a negative in
film photography: it contains untouched, "raw" pixel information straight from
the digital camera's
sensor. The RAW file format has yet to undergo
demosaicing,
and so it contains just one red, green, or blue value at each pixel location.
Digital cameras normally "develop" this RAW file by converting it into a full
color
JPEG or TIFF image file, and then store the converted file in your memory
card. Digital cameras have to make several interpretive decisions when
they develop a RAW file, and so the RAW file format offers you more control
over how the final JPEG or TIFF image is generated. This section aims
to illustrate the technical advantages of RAW files, and makes suggestions about
when to use the RAW file format.

OVERVIEW

A RAW file is developed into a final JPEG or TIFF image in several steps,
each of which may contain several irreversible image adjustments. One
key advantage of RAW is that it allows the photographer to postpone applying
these adjustments-- giving more flexibility to the photographer to later apply
these themselves, in a way which best suits each image. The following
diagram illustrates the sequence of adjustments:

Demosaicing and
white
balance involve interpreting and converting the
bayer array
into an image with all three colors at each pixel, and occur in the same step.
The bayer array is what makes the first image appear more pixelated than the
other two, and gives the image a greenish tint.

Our eyes perceive differences in lightness logarithmically, and so when light
intensity quadruples we only perceive this as a doubling in the amount of light.
A digital camera, on the other hand, records differences in lightness linearly--
twice the light intensity produces twice the response in the camera sensor.
This is why the first and second images above look so much darker than the third.
In order for the numbers recorded within a digital camera to be shown as we
perceive them, tone curves need to be applied.

Color saturation and
contrast may also be adjusted, depending on the setting within your camera.
The image is then sharpened to offset the softening caused by demosaicing, which
is visible in the second image.

The high bit depth RAW image is
then converted into 8-bits per channel, and compressed into a JPEG based on
the compression setting within your camera. Up until this step, RAW image
information most likely resided within the digital camera's memory buffer.

There are several advantages to performing any of the above RAW conversion
steps afterwards on a personal computer, as opposed to within a digital camera.
The next sections describe how using RAW files can enhance these RAW conversion
steps.

DEMOSAICING

Demosaicing
is a very processor-intensive step, and so the best demosaicing algorithms require
more processing power than is practical within today's digital cameras.
Most digital cameras therefore take quality-compromising shortcuts to convert
a RAW file into a TIFF or JPEG in a reasonable amount of time. Performing
the demosaicing step on a personal computer allows for the best algorithms since
a PC has many times more processing power than a typical digital camera.
Better algorithms can squeeze a little more out of your camera sensor by producing
more resolution, less noise, better small-scale color accuracy and reduced moiré.
Note the resolution advantage shown below:

JPEG
(in-camera)

RAW

Ideal

Images from actual camera tests with a Canon EOS 20D using
an ISO 12233 resolution test chart.
Differential between RAW and JPEG resolution may vary with camera model
and conversion software.

The in-camera JPEG image is not able to resolve lines as closely spaced as
those in the RAW image. Even so, a RAW file cannot achieve the ideal lines
shown, because the process of demosaicing always introduces some softening to
the image. Only sensors which capture all three colors at each pixel location
could achieve the ideal image shown at the bottom (such as Foveon-type sensors).

FLEXIBLE WHITE BALANCE

White balance is the process of removing unrealistic color casts, so that
objects which appear white in person are rendered white in your photo.
Color casts within JPEG images can often be removed in post-processing, but
at the cost of bit depth and color gamut. This is because the white
balance has effectively been set twice: once in RAW conversion and then again
in post-processing. RAW files give you the ability to set the white balance
of a photo *after* the picture has been taken-- without unnecessarily destroying
bits.

HIGH BIT DEPTH

Digital cameras actually record each color channel with more precision than
the 8-bits (256 levels) per channel used for JPEG images (see "Understanding
Bit Depth"). Most current cameras capture each color with 12-bits
of precision (212 = 4096 levels) per color channel, providing several
times more levels than could be achieved by using an in-camera JPEG. Higher
bit depth decreases the susceptibility to
posterization,
and increases your flexibility when choosing a
color space and in post-processing.

DYNAMIC RANGE & EXPOSURE COMPENSATION

The RAW file format usually provides considerably more "dynamic range" than
a JPEG file, depending on how the camera creates its JPEG. Dynamic range
refers to the range of light to dark which can be captured by a camera before
becoming completely white or black, respectively. Since the raw color
data has not been converted into logarithmic values using curves (see overview
section above), the exposure of a RAW file can be adjusted slightly-- after
the photo has been taken. Exposure compensation can correct for metering
errors, or can help bring out lost shadow or highlight detail. The following
example was taken directly into the setting sun, and shows the same RAW file
with -1 stop, 0 (no change), and +1 stop exposure compensation. Move your
mouse over each to see how exposure compensation affects the image:

Apply Exposure Compensation:

-1.0

none

+1.0

Note: +1 or -1 stop refers to a doubling or halving of the light
used for an exposure, respectively.

A stop can also be listed in terms of eV, and so +1 stop is
equivalent to +1 eV.

Note the broad range of shadow and highlight detail across the three images.
Similar results could not be achieved by merely brightening or darkening a JPEG
file-- both in dynamic range and in the smoothness of tones. A graduated
neutral density filter (see tutorial on
camera lens filters) could then be used to better utilize this broad dynamic
range.

ENHANCED SHARPENING

Since a RAW file is untouched, sharpening has not been applied within the
camera. Much like demosaicing, better sharpening algorithms are often
far more processor intensive. Sharpening performed on a personal computer
can thus create fewer halo artifacts for an equivalent amount of sharpening
(see "Sharpening
Using an Unsharp Mask" for examples of sharpening artifacts).

Since sharpness depends on the intended
viewing distance of your image, the RAW file format also provides more control
over what type and how much sharpening is applied (given your purpose).
Sharpening is usually the last post-processing step since it cannot be undone,
so having a pre-sharpened JPEG is not optimal.

LOSSLESS COMPRESSION

The RAW file format uses a lossless compression, and so it does not suffer
from the
compression artifacts visible with "lossy" JPEG compression. RAW files
contain more information and achieve better compression than TIFF, but without
the compression artifacts of JPEG.

Note: Kodak and Nikon employ a slightly lossy RAW compression algorithm,
although any artifacts are much lower than would be perceived with a similar
JPEG image. The efficiency of RAW compression also varies with digital
camera manufacturer.

DISADVANTAGES

RAW files are much larger than similar JPEG files, and so fewer photos
can fit within the same memory card.

RAW files are more time consuming since they may require manually applying
each conversion step.

RAW files often take longer to be written to a memory card since they
are larger, therefore most digital cameras may not achieve the same frame
rate as with JPEG.

RAW files cannot be given to others immediately since they require specific
software to load them, therefore it may be necessary to first convert them
into JPEG.

RAW files require a more powerful computer with more temporary memory
(RAM).

OTHER CONSIDERATIONS

One problem with the RAW file format is that it is not very standardized.
Each camera has their own proprietary RAW file format, and so one program may
not be able to read all formats. Fortunately, Adobe has announced a digital
negative (DNG) specification which aims to standardize the RAW file format.
In addition, any camera which has the ability to save RAW files should come
with its own software to read them.

Good RAW conversion software can perform batch processes and often automates
all conversion steps except those which you choose to modify. This can
mitigate or even eliminate the ease of use advantage of JPEG files.

Many newer cameras can save both RAW and JPEG images simultaneously.
This provides you with an immediate final image, but retains the RAW "negative"
just in case more flexibility is desired later.

SUMMARY

So which is better: RAW or JPEG? There is no single answer, as this depends
on the type of photography you are doing. In most cases, RAW files will
provide the best solution due to their technical advantages and the decreasing
cost of large memory cards. RAW files give the photographer far more control,
but with this comes the trade-off of speed, storage space and ease of use.
The RAW trade-off is sometimes not worth it for sports and press photographers,
although landscape and most fine art photographers often choose RAW in order
to maximize the image quality potential of their digital camera.